Police later arrested far-right supporter Thomas Mair, 52, who has a history of mental illness.

Yui Mok/PA WirePolice were reportedly set to increase security around murdered MP Jo Cox following a three-month hate mail campaign

The Times reported Friday that Cox, 41, had been harassed by a stream of messages in recent months and as a result police were looking at - but were yet to - increase security at her surgery and at her houseboat in London.

The newspaper said there was no link between the hate mail and Thursday's killing, which was the first murder of an MP in more than 25 years. Ian Gow, a parliamentary private secretary to Margaret Thatcher, was killed in 1990 when the IRA exploded a bomb under his car, outside his home in East Sussex.

Cox reportedly "thought little of" the hate mail until the volume and frequency increased, a source close to Cox's office told the Times. A man was said to have been cautioned over the communications.

Nigel Roddis/PA WireForensics officers at the scene of the murder in Birstall, West Yorkshire

MPs are said to have expressed concerns about security at their surgeries since Labour MP Stephen Timms was attacked by an Islamic fanatic in 2010. A report for the House of Commons Administration Committee last August revealed that security was a particular concern for women MPs, Sky News reported.

In the wake of the murder, Sky News reports that MPs have been told to contact police to review their security, with Tory chief whip Mark Harper and Labour counterpart Dame Rosie Winterton, issuing the advice.

Yui Mok/PA WireCox was said to have "thought little of" the hate mail until the volume and frequency increased

Conservative MP James Berry told the broadcaster the chief whip had given "us very similar advice" to the Labour chief whip.